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Two photon induced luminescence of BSA protected gold clusters
Chemical Physics Letters 561–562 (2013) 74–76
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Two photon induced luminescence of BSA protected gold clusters Sangram L. Raut a,⇑, Dmytro Shumilov b, Rahul Chib a, Ryan Rich a, Zygmunt Gryczynski a,b, Ignacy Gryczynski a,c,⇑ a
Center for Commercialization of Fluorescence Technologies, University of North Texas Health Science Center, 3500 Camp Bowie Blvd., Fort Worth, TX 76107, USA Department of Physics, Texas Christian University, 2800 S. University Dr., Fort Worth, TX 76129, USA c Department of Cell Biology and Anatomy, University of North Texas Health Science Center, 3500 Camp Bowie Blvd., Fort Worth, TX 76107, USA b
a r t i c l e
i n f o
Article history: Received 13 December 2012 In final form 20 January 2013 Available online 25 January 2013
a b s t r a c t In this short Letter, we have synthesized the BSA protected Au25 nanoclusters and studied their two photon luminescence behavior. We demonstrate that BSA Au25 nanoclusters can be used as a probe with two photon excitation capability. Our results show a quadratic relation between excitation power and emission intensity whereas with one photon excitation shows a linear dependence. The emission spectrum of BSA Au25 nanoclusters with one photon and two photon excitation shows no appreciable change. Due to its long wavelength emission (650 nm) and two photon excitation, BSA Au25 can be potentially used as a probe for deep tissue imaging. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The problems associated with high sensitivity cellular imaging through tissue have stimulated efforts in synthesizing and using red and near-infrared (NIR) fluorophores that can be excited within the optical window from 630 to 1100 nm [1–4]. Commercial manufacturers like Molecular Probes, Pierce, and SETA Biomedicals have synthesized a newer class of IR dyes that have several fold higher fluorescence signal to auto-fluorescence noise ratio compared to traditional IR dyes like Cy 5 and Indocyanine green [5–7]. Two photon excitation (2PE) of fluorescence offers high spatial resolution due to its quadratic intensity dependence. Increased use of 2PE induced fluorescence in microscopy and imaging can be well justified by its ability to localize the excitation to a focal point and only excite fluorophores present there [8–12]. Out of focus fluorophores are not excited and hence they are not photobleached. Moreover, long wavelengths (>1000 nm) excitations results in less interference from the auto-fluorescence signal owing to less infrared (IR) absorption, scattering and fluorescence from tissue [13–15]. Currently, considerable research efforts are directed towards synthesizing metal nanoclusters (Au, Ag and Cu) due to their unique photo-physical properties and possible application in cellular and tissue imaging [16–21]. Protein protected, especially bovine serum albumin (BSA), Au clusters has been extensively characterized for one photon spectroscopic properties (red emission, about ⇑ Corresponding authors. Address: Center for Commercialization of Fluorescence Technologies, University of North Texas Health Science Center, 3500 Camp Bowie Blvd., Fort Worth, TX 76107, USA. E-mail addresses: [email protected] (S.L. Raut), Ignacy.gryczynski@unthsc. edu (I. Gryczynski). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.01.028
6–7% quantum efficiency and photo-stable), metal ion sensing and cellular imaging [22–25]. Despite these reports, little is known about 2PE luminescence of these BSA Au25 clusters. Previously, Patel et al. have reported 2PE luminescence properties of oligonuleotide protected Ag nanoclusters having a large two photon absorption cross section [26]. In another report, Ramakrishna et al. show that the 2PE luminescence of alkyl thiolate protected gold clusters dependent on cluster size [27]. In this Letter we aim to show that BSA Au25 clusters also exhibit an efficient two photon absorption followed by red to near-infrared luminescence. We think that information regarding spectral properties of BSA protected Au clusters with 2PE is of interest for its application to biophysics and time resolved fluorescence. 2. Experimental section 2.1. Synthesis of BSA Au25 nanoclusters The Au25 NCs used in this Letter were synthesized using an approach developed by Xie et al. [23]. Typically, 5 mL of 10 mM HAuCl4 was mixed with 5 mL of 50 mg/mL BSA with 0.5 mL of 1 M NaOH and kept at 37 °C overnight in the incubator. The light brown solution of clusters was further dialyzed (2000 MWCO membrane) against de-ionized water for at least 12 h with periodic change of water to remove any small impurities. Dialyzed cluster solution was filtered using 0.02 lm syringe filter and used for subsequent measurements. 2.2. Spectroscopic measurements UV–Vis absorption and luminescence spectra (one photon) were obtained using a Cary 50 bio UV–Visible spectrophotometer
S.L. Raut et al. / Chemical Physics Letters 561–562 (2013) 74–76
75
(Varian Inc.) and Cary Eclipse spectrofluorometer (Varian Inc.) respectively. All the measurements were done in 1 0.4 cm cuvettes at room temperature. A solution of rhodamine B (RhB) in water was used as a reference for comparison. Two photon excitation was achieved with Origami-10, 1038 nm pulsed laser (40 MHz repetition rate, 180 mW average power and 140 fs pulse duration) and 10 objective, mounted on a horizontal positioner. Collected emission was analyzed with a FT300 (Picoquant GmbH. Germany) fluorescence lifetime spectrofluorometer. The hydrodynamic particle size of BSA Au25 clusters was measured using a Nanotrac system (Mircotrac, Inc., Montgomeryville, PA, USA) in water at room temperature. 3. Results and discussion Figure 1 show the cartoon of molecular structure of the BSA protected Au25 clusters and measured hydrodynamic size. Although narrow, there is a size distribution with an average size of 8 nm. This observation points towards the presence of multiple BSA molecules protecting each Au cluster. In Figure 2, the red line shows the corrected absorption spectrum of the BSA Au25 clusters. The measured extinction spectrum of the clusters consists of absorption and Rayleigh scattering (1/k4) components. The scattering component was later subtracted from the total extinction spectrum in order to obtain corrected absorption of clusters. The blue line shows the absorption of RhB in water. These two solutions were used for subsequent one and two photon experiments. Figure 3, upper panel, shows a photograph in which the cuvette on the left demonstrates one photon excitation of the BSA Au25 solution, excited using the second harmonic of a 1038 nm laser. The cuvette on the right shows two photon excitation of the BSA Au25 solution using a 1038 nm laser with a focusing lens in the light path. A red filter (560 nm) was used in the excitation light path to avoid transmitting green light. It is evident from the photograph that we can achieve selective and strong excitation at the focal point using invisible near-infrared 1038 nm light. Furthermore, we looked at the luminescence spectra of BSA Au25
Figure 2. Absorption spectra of BSA Au25 (red line) and rhodamine B (blue line). These solutions were used for 2 photon experiments. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Figure 3. Upper panel: left cuvette shows the one photon excitation of BSA Au25 solution excited using second harmonic of 1038 nm laser and right cuvettee shows two photon excitation of BSA Au25 solution using 1038 nm laser. Lower panel one photon and two photon emission spectra of BSA Au25 clusters in water at room temperature using 519 nm and 1038 nm laser, respectively.
Figure 1. Cartoon of BSA protected Au25 clusters and hydrodynamic particle size of the BSA Au25.
clusters in one photon and two photon excitation conditions and found that there is no appreciable difference between them (Figure 3 lower panel). The luminescence emission intensity response to the excitation power is different for one and two photon excitations. The relation between excitation power and emission intensity for one photon excitation is presented in Figure 4A which shows linear relationship between excitation power and emission intensity. However, Figure 4B shows a quadratic dependence between excitation power and emission intensity. Figure 5 shows the 2PE induced luminescence spectra of BSA Au25 clusters and RhB fluorescence in water. Similar instrument settings were used for clusters and RhB. The solutions that are used
76
S.L. Raut et al. / Chemical Physics Letters 561–562 (2013) 74–76
A
in Figure 1 were used for these measurements. It is difficult to calculate the two photon absorption cross section due to the difficulty in calculating the precise concentration of clusters in the solution. BSA Au25 clusters are composed of more than one molecule of BSA and 25 gold atoms which makes it complex to calculate the exact mass of the complex and hence the concentration. Finding the exact number of BSA molecules in the cluster complex needs further studies. Nonetheless, one can get a qualitative estimation of the two photon absorption cross section of BSA Au25 clusters from the comparison with RhB absorption and fluorescence. 4. Conclusions
B
In this Letter we have synthesized BSA Au25 clusters using the established protocol, and we have confirmed their absorption and luminescence properties. The major observation of our study is the 2PE ability of BSA Au25 clusters. The presence of 2PE properties among these clusters opens up a door to many exciting applications in microscopy and time resolved fluorescence. Moreover, being in the NIR region together with 2PE capability will make them ideal imaging candidate. Acknowledgement This work was supported by the NIH Grant R01EB12003. References [1] [2] [3] [4] [5] [6]
Figure 4. (A) One photon emission intensity dependence on excitation power of BSA Au25 clusters showing linear relation. (B) Two photon emission intensity dependence on excitation power of BSA Au25 clusters showing quadratic relation.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Figure 5. Two photon emission spectra of RhB (blue line) and BSA Au25 clusters in water at room temperature using 1038 nm laser. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[26] [27]
W. Pham, Z. Medarova, A. Moore, Bioconjugate Chem. 16 (2005) 735. Y. Shibata, J.B. Kruskal, M.R. Palmer, J. Neurosci. Methods 147 (2005) 82. W.S. El-Deiry, C.C. Sigman, G.J. Kelloff, J. Clin. Oncol. 24 (2006) 3261. M. Bai, S. Achilefu, Bioorg. Med. Chem. Lett. 21 (2011) 280. L. Patsenker, A. Tatarets, O. Kolosova, O. Obukhova, Y. Povrozin, I. Fedyunyayeva, I. Yermolenko, et al., Ann. NY Acad. Sci. 1130 (2008) 179. T. Steinberg, J. Nyhus, B. Aggeler, L. Greenfield, L. Leong, Cambridge Healthtech Institute’s Third Annual IN VIVO MOLECULAR IMAGING: Moving from Discovery to Pre-Clinical to Clinical Applications, 2006. www.Piercenet.com. Accessed on 3rd Dec 2012. A. Periasamy, P. Skoglund, C. Noakes, R. Keller, Microsc. Res. Tech. 47 (1999) 172. S.S. Vogel, C. Thaler, P.S. Blank, S.V. Koushik, FLIM Microsc. Biol. Med. 1 (2009) 245. C. Xu, W.W. Webb, JOSA B 13 (1996) 481. D.W. Piston, Trends Cell Biol. 9 (1999) 66. M. Schneider, S. Barozzi, I. Testa, M. Faretta, A. Diaspro, Biophys. J. 89 (2005) 1346. J. Qian et al., Angew. Chem., Int. Ed. Engl. 51 (2012) 10570. J. Geng et al., Small 8 (2012) 3655–3663. J. Lakowicz (Ed.), Topics in Fluorescence Spectroscopy, Plenum Press, New York, 1997. S. Raut et al., Chem. Phys. Lett. 549 (2012) 72. H. Qian, M. Zhu, Z. Wu, R. Jin, Acc. Chem. Res. 45 (2012) 1470. H. Xu, K.S. Suslick, Adv. Mater. 22 (2010) 1078. Y. Lu, W. Chen, Chem. Soc. Rev. 41 (2012) 3594. A. Retnakumari et al., Nanotechnology 21 (2010) 055103. C.V. Durgadas, C.P. Sharma, K. Sreenivasan, Analyst 136 (2011) 933. X.X. Wang, Q. Wu, Z. Shan, Q.M. Huang, Biosens. Bioelectron. 26 (2011) 3614. J. Xie, Y. Zheng, J.Y. Ying, J. Am. Chem. Soc. 131 (2009) 888. L. Hu, S. Han, S. Parveen, Y. Yuan, L. Zhang, G. Xu, Biosens. Bioelectron. 32 (2012) 297. T. Das, P. Ghosh, M.S. Shanavas, A. Maity, S. Mondal, P. Purkayastha, Nanoscale 4 (2012) 6018. S.A. Patel, C.I. Richards, J.C. Hsiang, R.M. Dickson, J. Am. Chem. Soc. 130 (2008) 11602. G. Ramakrishna, O. Varnavski, J. Kim, D. Lee, T. Goodson, J. Am. Chem. Soc. 130 (2008) 5032.
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Two photon induced luminescence of BSA protected gold clusters Sangram L. Raut a,⇑, Dmytro Shumilov b, Rahul Chib a, Ryan Rich a, Zygmunt Gryczynski a,b, Ignacy Gryczynski a,c,⇑ a
Center for Commercialization of Fluorescence Technologies, University of North Texas Health Science Center, 3500 Camp Bowie Blvd., Fort Worth, TX 76107, USA Department of Physics, Texas Christian University, 2800 S. University Dr., Fort Worth, TX 76129, USA c Department of Cell Biology and Anatomy, University of North Texas Health Science Center, 3500 Camp Bowie Blvd., Fort Worth, TX 76107, USA b
a r t i c l e
i n f o
Article history: Received 13 December 2012 In final form 20 January 2013 Available online 25 January 2013
a b s t r a c t In this short Letter, we have synthesized the BSA protected Au25 nanoclusters and studied their two photon luminescence behavior. We demonstrate that BSA Au25 nanoclusters can be used as a probe with two photon excitation capability. Our results show a quadratic relation between excitation power and emission intensity whereas with one photon excitation shows a linear dependence. The emission spectrum of BSA Au25 nanoclusters with one photon and two photon excitation shows no appreciable change. Due to its long wavelength emission (650 nm) and two photon excitation, BSA Au25 can be potentially used as a probe for deep tissue imaging. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The problems associated with high sensitivity cellular imaging through tissue have stimulated efforts in synthesizing and using red and near-infrared (NIR) fluorophores that can be excited within the optical window from 630 to 1100 nm [1–4]. Commercial manufacturers like Molecular Probes, Pierce, and SETA Biomedicals have synthesized a newer class of IR dyes that have several fold higher fluorescence signal to auto-fluorescence noise ratio compared to traditional IR dyes like Cy 5 and Indocyanine green [5–7]. Two photon excitation (2PE) of fluorescence offers high spatial resolution due to its quadratic intensity dependence. Increased use of 2PE induced fluorescence in microscopy and imaging can be well justified by its ability to localize the excitation to a focal point and only excite fluorophores present there [8–12]. Out of focus fluorophores are not excited and hence they are not photobleached. Moreover, long wavelengths (>1000 nm) excitations results in less interference from the auto-fluorescence signal owing to less infrared (IR) absorption, scattering and fluorescence from tissue [13–15]. Currently, considerable research efforts are directed towards synthesizing metal nanoclusters (Au, Ag and Cu) due to their unique photo-physical properties and possible application in cellular and tissue imaging [16–21]. Protein protected, especially bovine serum albumin (BSA), Au clusters has been extensively characterized for one photon spectroscopic properties (red emission, about ⇑ Corresponding authors. Address: Center for Commercialization of Fluorescence Technologies, University of North Texas Health Science Center, 3500 Camp Bowie Blvd., Fort Worth, TX 76107, USA. E-mail addresses: [email protected] (S.L. Raut), Ignacy.gryczynski@unthsc. edu (I. Gryczynski). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.01.028
6–7% quantum efficiency and photo-stable), metal ion sensing and cellular imaging [22–25]. Despite these reports, little is known about 2PE luminescence of these BSA Au25 clusters. Previously, Patel et al. have reported 2PE luminescence properties of oligonuleotide protected Ag nanoclusters having a large two photon absorption cross section [26]. In another report, Ramakrishna et al. show that the 2PE luminescence of alkyl thiolate protected gold clusters dependent on cluster size [27]. In this Letter we aim to show that BSA Au25 clusters also exhibit an efficient two photon absorption followed by red to near-infrared luminescence. We think that information regarding spectral properties of BSA protected Au clusters with 2PE is of interest for its application to biophysics and time resolved fluorescence. 2. Experimental section 2.1. Synthesis of BSA Au25 nanoclusters The Au25 NCs used in this Letter were synthesized using an approach developed by Xie et al. [23]. Typically, 5 mL of 10 mM HAuCl4 was mixed with 5 mL of 50 mg/mL BSA with 0.5 mL of 1 M NaOH and kept at 37 °C overnight in the incubator. The light brown solution of clusters was further dialyzed (2000 MWCO membrane) against de-ionized water for at least 12 h with periodic change of water to remove any small impurities. Dialyzed cluster solution was filtered using 0.02 lm syringe filter and used for subsequent measurements. 2.2. Spectroscopic measurements UV–Vis absorption and luminescence spectra (one photon) were obtained using a Cary 50 bio UV–Visible spectrophotometer
S.L. Raut et al. / Chemical Physics Letters 561–562 (2013) 74–76
75
(Varian Inc.) and Cary Eclipse spectrofluorometer (Varian Inc.) respectively. All the measurements were done in 1 0.4 cm cuvettes at room temperature. A solution of rhodamine B (RhB) in water was used as a reference for comparison. Two photon excitation was achieved with Origami-10, 1038 nm pulsed laser (40 MHz repetition rate, 180 mW average power and 140 fs pulse duration) and 10 objective, mounted on a horizontal positioner. Collected emission was analyzed with a FT300 (Picoquant GmbH. Germany) fluorescence lifetime spectrofluorometer. The hydrodynamic particle size of BSA Au25 clusters was measured using a Nanotrac system (Mircotrac, Inc., Montgomeryville, PA, USA) in water at room temperature. 3. Results and discussion Figure 1 show the cartoon of molecular structure of the BSA protected Au25 clusters and measured hydrodynamic size. Although narrow, there is a size distribution with an average size of 8 nm. This observation points towards the presence of multiple BSA molecules protecting each Au cluster. In Figure 2, the red line shows the corrected absorption spectrum of the BSA Au25 clusters. The measured extinction spectrum of the clusters consists of absorption and Rayleigh scattering (1/k4) components. The scattering component was later subtracted from the total extinction spectrum in order to obtain corrected absorption of clusters. The blue line shows the absorption of RhB in water. These two solutions were used for subsequent one and two photon experiments. Figure 3, upper panel, shows a photograph in which the cuvette on the left demonstrates one photon excitation of the BSA Au25 solution, excited using the second harmonic of a 1038 nm laser. The cuvette on the right shows two photon excitation of the BSA Au25 solution using a 1038 nm laser with a focusing lens in the light path. A red filter (560 nm) was used in the excitation light path to avoid transmitting green light. It is evident from the photograph that we can achieve selective and strong excitation at the focal point using invisible near-infrared 1038 nm light. Furthermore, we looked at the luminescence spectra of BSA Au25
Figure 2. Absorption spectra of BSA Au25 (red line) and rhodamine B (blue line). These solutions were used for 2 photon experiments. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Figure 3. Upper panel: left cuvette shows the one photon excitation of BSA Au25 solution excited using second harmonic of 1038 nm laser and right cuvettee shows two photon excitation of BSA Au25 solution using 1038 nm laser. Lower panel one photon and two photon emission spectra of BSA Au25 clusters in water at room temperature using 519 nm and 1038 nm laser, respectively.
Figure 1. Cartoon of BSA protected Au25 clusters and hydrodynamic particle size of the BSA Au25.
clusters in one photon and two photon excitation conditions and found that there is no appreciable difference between them (Figure 3 lower panel). The luminescence emission intensity response to the excitation power is different for one and two photon excitations. The relation between excitation power and emission intensity for one photon excitation is presented in Figure 4A which shows linear relationship between excitation power and emission intensity. However, Figure 4B shows a quadratic dependence between excitation power and emission intensity. Figure 5 shows the 2PE induced luminescence spectra of BSA Au25 clusters and RhB fluorescence in water. Similar instrument settings were used for clusters and RhB. The solutions that are used
76
S.L. Raut et al. / Chemical Physics Letters 561–562 (2013) 74–76
A
in Figure 1 were used for these measurements. It is difficult to calculate the two photon absorption cross section due to the difficulty in calculating the precise concentration of clusters in the solution. BSA Au25 clusters are composed of more than one molecule of BSA and 25 gold atoms which makes it complex to calculate the exact mass of the complex and hence the concentration. Finding the exact number of BSA molecules in the cluster complex needs further studies. Nonetheless, one can get a qualitative estimation of the two photon absorption cross section of BSA Au25 clusters from the comparison with RhB absorption and fluorescence. 4. Conclusions
B
In this Letter we have synthesized BSA Au25 clusters using the established protocol, and we have confirmed their absorption and luminescence properties. The major observation of our study is the 2PE ability of BSA Au25 clusters. The presence of 2PE properties among these clusters opens up a door to many exciting applications in microscopy and time resolved fluorescence. Moreover, being in the NIR region together with 2PE capability will make them ideal imaging candidate. Acknowledgement This work was supported by the NIH Grant R01EB12003. References [1] [2] [3] [4] [5] [6]
Figure 4. (A) One photon emission intensity dependence on excitation power of BSA Au25 clusters showing linear relation. (B) Two photon emission intensity dependence on excitation power of BSA Au25 clusters showing quadratic relation.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Figure 5. Two photon emission spectra of RhB (blue line) and BSA Au25 clusters in water at room temperature using 1038 nm laser. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[26] [27]
W. Pham, Z. Medarova, A. Moore, Bioconjugate Chem. 16 (2005) 735. Y. Shibata, J.B. Kruskal, M.R. Palmer, J. Neurosci. Methods 147 (2005) 82. W.S. El-Deiry, C.C. Sigman, G.J. Kelloff, J. Clin. Oncol. 24 (2006) 3261. M. Bai, S. Achilefu, Bioorg. Med. Chem. Lett. 21 (2011) 280. L. Patsenker, A. Tatarets, O. Kolosova, O. Obukhova, Y. Povrozin, I. Fedyunyayeva, I. Yermolenko, et al., Ann. NY Acad. Sci. 1130 (2008) 179. T. Steinberg, J. Nyhus, B. Aggeler, L. Greenfield, L. Leong, Cambridge Healthtech Institute’s Third Annual IN VIVO MOLECULAR IMAGING: Moving from Discovery to Pre-Clinical to Clinical Applications, 2006. www.Piercenet.com. Accessed on 3rd Dec 2012. A. Periasamy, P. Skoglund, C. Noakes, R. Keller, Microsc. Res. Tech. 47 (1999) 172. S.S. Vogel, C. Thaler, P.S. Blank, S.V. Koushik, FLIM Microsc. Biol. Med. 1 (2009) 245. C. Xu, W.W. Webb, JOSA B 13 (1996) 481. D.W. Piston, Trends Cell Biol. 9 (1999) 66. M. Schneider, S. Barozzi, I. Testa, M. Faretta, A. Diaspro, Biophys. J. 89 (2005) 1346. J. Qian et al., Angew. Chem., Int. Ed. Engl. 51 (2012) 10570. J. Geng et al., Small 8 (2012) 3655–3663. J. Lakowicz (Ed.), Topics in Fluorescence Spectroscopy, Plenum Press, New York, 1997. S. Raut et al., Chem. Phys. Lett. 549 (2012) 72. H. Qian, M. Zhu, Z. Wu, R. Jin, Acc. Chem. Res. 45 (2012) 1470. H. Xu, K.S. Suslick, Adv. Mater. 22 (2010) 1078. Y. Lu, W. Chen, Chem. Soc. Rev. 41 (2012) 3594. A. Retnakumari et al., Nanotechnology 21 (2010) 055103. C.V. Durgadas, C.P. Sharma, K. Sreenivasan, Analyst 136 (2011) 933. X.X. Wang, Q. Wu, Z. Shan, Q.M. Huang, Biosens. Bioelectron. 26 (2011) 3614. J. Xie, Y. Zheng, J.Y. Ying, J. Am. Chem. Soc. 131 (2009) 888. L. Hu, S. Han, S. Parveen, Y. Yuan, L. Zhang, G. Xu, Biosens. Bioelectron. 32 (2012) 297. T. Das, P. Ghosh, M.S. Shanavas, A. Maity, S. Mondal, P. Purkayastha, Nanoscale 4 (2012) 6018. S.A. Patel, C.I. Richards, J.C. Hsiang, R.M. Dickson, J. Am. Chem. Soc. 130 (2008) 11602. G. Ramakrishna, O. Varnavski, J. Kim, D. Lee, T. Goodson, J. Am. Chem. Soc. 130 (2008) 5032.